The present invention relates to a cast iron material which exhibits good welding and workability characteristics.
In order to produce a cast iron material which exhibits good welding and good workability properties for activities such as cutting, the cast iron material is subjected to decarburization and annealing. Traditionally, it has also been necessary to include so-called "chip-breakers" in the cast iron material. In malleable cast iron having these properties, manganese sulfides (MnS) and temper carbon are often used as the chip-breakers. After treating the cast iron material in the above manner, the iron casting thereby obtained may then be welded, cut, etc.
A number of different procedures have been used in the prior art to obtain a cast iron material having good workability properties. However, these methods have produced relatively unsatisfactory results.
According to one procedure, a raw or undressed iron casting which also contains 2.7-3.0% carbon (C), 0.25-0.35% silicon (Si), 0.70-0.85% manganese (Mn), 0.02-0.06% sulphur (S), and customary proportions of phosphorus (P) is oxidized and annealed at a temperature in the range of 1020.degree. C.-1070.degree. C. The nonmagnetic solid solution of ferritic carbide present is removed during its austenitic phase. The iron carbide therefore does not decompose to temper carbon. The ferritic carbide which is removed, forms a ferritic or ferritic-pearlitic structure because of the relatively low silicon and sulphur content of the raw material. Accordingly, a large excess of manganese is present.
This procedure has a serious drawback in that once a ferritic or ferritic-pearlitic structure is formed, the composition must then be adapted for its use as a weldable and malleable cast iron. For example, the sulphur content of the resultant composition (from 0.02-0.06%) is considerably higher than the amount customarily included in a composition exhibiting good welding characteristics. Therefore, in order to impart good welding characteristics to this composition, it is necessary to desulphurize the melt, a procedure which is extremely expensive.
In another procedure, a malleable cast iron having a standard composition, namely, 2.0-3.0% C, 0.6-1.5% Si, and less than 0.6% Mn, 0.15% P, and 0.15% S, respectively, is used as the starting material. The cast iron is subjected to decarburizing-annealing at a temperature between 950.degree. C.-1100.degree. C. The malleable cast iron material obtained in this procedure, as well as the procedures discussed above, solidifies in the mold in a graphite-free manner, the carbon being bound in the form of Fe.sub.3 C (carbide).
Other procedures have employed black heart malleable iron to obtain a material with the desired properties. In one such procedure, black heart malleable iron is treated with pure magnesium. The casting is then annealed in a stepwise manner at about 700.degree. C.-900.degree. C. However, the resultant material cannot be used for construction weldings with high stress or high load requirements because high concentrations of graphite carbon are present in the generally ferritic base mass. These high concentrations of graphite carbon cause strain upon hardening of the composition and blister formation.
More recently, it has been proposed to render nodular cast iron, or iron which contains spheroidal graphite inclusions, suitable for welding, cutting, etc. It is well known in the art that through appropriate magnesium treatment, the uncombined carbon in the cast iron melt take the compact spheroidal form which is characteristic of nodular cast iron. Nodular cast iron possess greatly improved mechanical properties of the castings in addition to other different properties than that found in grey cast iron.
A nodular cast iron which exhibits good welding characteristics is known from Swiss patent 496,098. The desired characteristics are obtained by subjecting the nodular cast iron pieces to a decarburizing-annealing phase which must be effected within a temperature range between 900.degree. C.-1200.degree. C. The nodular cast iron which is used contains about 2.4-3.4% C, 0.4-2.4% Si, 0.1-0.7% Mn, 0.07% P, 0.005% or less S, and a residual Mg content of 0.02-0.10% by weight, the remainder being iron. The decarburization is carried out until the desired carbon content in the casting has been obtained. As an additional step, the magnesium-treated melt is inoculated with small amounts of ferrosilicon at a terminal stage in order to counteract the susceptibility to hard cracking. The resultant material has a decreased carbon content at least in the welding zones. The majority of the carbon present within the cast iron after treatment is in the form of spheroidal graphite, in contrast to the Fe.sub.3 C found in the procedures outlined above.
However, the process disclosed in the Swiss patent is also unfavorable because of the high Si content of the cast iron material. The high Si content causes the undesirable and uncontrollable production of FeO.
It is therefore an object of the present invention to provide a process for the production of a cast iron material which possesses good material qualities while providing a nearly-faultless surface with good workability characteristics.
It is another object of the present invention to provide a process for the production of a nodular cast iron material which does not require the addition of MnS and temper carbon as chip-breakers.
It is a further object of the present invention to provide a process for the production of a material which employs nodular cast iron and which does not result in the undesirable production of FeO.
It is yet another object of the present invention to provide a process and material which is characterized by good weldability and at the same time by good cutting properties.